Recently, accompanied with the progress in the digital technology, image formation by digital system becomes main stream of the image forming method by electrophotographic system. The digital image forming method is based on the imaging of small dot image of one pixel such as 1200 dpi (dpi is number of the dot per inch or 2.54 cm). Therefore, technique suitable for exactly reproducing a high quality image is demanded.
From such the viewpoint of rising in the image quality, miniaturization of the toner particle is progresses. Hitherto, so called crushed toner is principally employed for Formation of an electrophotographic image, which is prepared by mixing and kneading binder resin and pigment and crushing and classifying the crushed toner powder. However, there is limit to miniaturization of the toner particle and unifying the particle size distribution. Accordingly, sufficient high image quality is difficultly attained in the image employing the crushed toner.
Recently, polymerized toner prepared by suspension polymerization or emulsion polymerization is noted as the means for attaining the miniaturization of the particle and for unifying the size distribution and the shape of the toner.
The polymerization method of the toner include a method in which resin particle and, according to necessity, colorant particles are associated or salted out/fused to prepare toner particles having irregular shape, and a method in which colorant is mixed and dispersed in radical polymerizable monomer and the resultant dispersion is dispersed in a liquid to form oil droplets having a designate diameter and then the oil droplets are subjected to suspension polymerization. Among them, the former polymerization method is preferable for forming the irregular shaped toner. In this polymerization method, a water-soluble polymerization initiator is employed for polymerization. On this occasion, a chain-transfer agent is employed for controlling the molecular weight distribution.
However, volatile components contained in the polymerizable monomer or the chain-transfer agent is difficultly removed completely from the toner particles on the occasion of the production.
The toner containing large amount of the volatile tends to be aggregated and a developer using the aggregated toner tends to cause problems such as that the image quality is degraded on the occasion of the image formation and sufficient quality image cannot be obtained, bad odor occurs on the occasion of fixing by evaporation of the volatile substance and the polymerizable monomer remaining in the toner, and the printed surfaces of image receiving sheets such as paper adhere with together on the occasion of high speed printing on the both sides.
The problems caused by the polymerizable monomer and the chain-transfer agent remaining in the toner are not specifically appeared as an important matter in the crashed toner prepared by melting, kneading and crushing the binder and the colorant. The reason of that is considered that the binder resin to be employed in the crushed toner is previously dried in many cases and the volatile component is removed by heating in the melting and kneading process of the production if the resin contains the volatile substance such as the unreacted polymerizable monomer.
In the polymerized polymer, however, it is considered that the unreacted monomer and the volatile substance can not be completely removed and the above problems are caused by the remaining volatile components since the melting and kneading process is not included in the production processes.
As to such the problems, a method by prescribing the amount of styrene monomer remaining in the toner and a method by prescribing the amount of remaining monomer have been disclosed. However, the problems of the occurrence of bad odor on the occasion of thermal fixing and that of the tacking of the printed surfaces of the image receiving sheets on the occasion of high speed printing on both sides of the sheet can not be solved and the property as the printing method is insufficient (see Patent documents 1 and 2).                Patent document 1: Japanese Patent Publication Open to Public Inspection, hereinafter referred to as Japanese Patent O.P.I. Publication, No. 2002-251037        Patent document 2: Japanese Patent O.P.I. Publication No. 2002-49176        